1. Field of the Invention
The present invention relates generally to switching of optical signals; and in particular, to the spatial routing of optical signals transmitted in optical communication networks and optical signal processing.
2. Statement of the Problem
Optical fibers are used as the physical media for transmitting optical signals in a variety of commercial and military applications. As the data rates of information continue to grow, it becomes increasingly difficult for conventional electronic switching systems to handle higher bandwidths. in addition, the required conversion between optical and electrical signals restricts the data format and increases costs. All-optical routing/switching technologies, characterized by high "data transparency," can switch or transfer optical signals from one transmission channel to another while the signals remain in optical form.
Several multiplexing schemes have been developed in fiber optic interconnection networks, including time-division multiplexing (TDM), wavelength-division multiplexing (WDM) and space-division multiplexing (SDM). Space-division switching is considered to be one of the most important fiber optic routing schemes. Major applications of space-division photonic switches are in fiber optic communication networks, optical gyroscopes, optical signal processing, and micro/millimeter wave signal distribution for phased-array radar systems.
Optical systems have been developed to provide programmable routing schemes that can alter the path of an optical data stream without effecting the characteristics of the data stream. A wide variety of electromagnetic field-controlled optical switches are commercially available. They are based on mechanical, electro-optic, thermo-optic, acousto-optic, magneto-optic, and semiconductor technologies. Each switching technology has its own advantages, but also has drawbacks as well. For example, mechanical switches are the most widely used routing components and provide very low insertion loss and cross-talk characteristics, but their switching time is limited to the millisecond range. They also have a limited lifetime because motor-driven parts are used. LiNbO.sub.3 integrated optic switches, on the other hand, offer nanosecond switching times. However, LiNbO.sub.3 switches suffer from the disadvantages of relative large insertion loss (5 dB), high cross-talk (20 dB) and polarization dependency.
Polarization-based optical routing switches has been disclosed in the past. For example, Chorum Technologies, Inc. of Richardson, Tex., has developed devices using polarizers, waveplates, and liquid crystals to alter the polarization state of the light stream that allow for routing the light stream into different optical fibers. These devices have low insertion loss, polarization mode dispersion, crosstalk, and polarization dependent loss. The switching speed is comparable to that of mechanical switches. Designs based on cubic and rectangular polarizing beamsplitters have been produced at Chorum Technologies, Inc. to create 1.times.2, 1.times.8, 1.times.64, and add/drop switches.
Polarized beamsplitters have also been used in the past in optical routing switches. A polarized beamsplitter consists of two prisms epoxied together with one of the hypotenii having a thin-film PBS coating with alternating high and low index materials. This component reflects incoming light at a 90.degree. angle that has one linear polarization state (S polarization) while allowing the orthogonal polarization state (P polarization) to pass through the hypotenuse coating.
This concept has been extended by Nitto Optical Corporation of Japan by joining four prisms to create an X-shaped interface between the prisms with a PBS coating. This X-cube device is used by Nitto Optical Corp. as an optical component in desktop projectors. In particular, the Nitto device is employed as a three-color combiner in video projectors. By controlling the polarization state of the individual color pixels, the intensity and color to the projected image can be controlled.
One example of a polarization-based optical routing switch is shown in U.S. Pat. No. 5,381,250 (Meadows). Meadows discloses a four-port optical routing switch in which four prisms are joined to form an X-shaped interface between the prisms. A polarizing beamsplitter (PBS) film is applied to the interface surfaces to selectively route light based on its polarization. Light having a first polarization is transmitted, while orthogonally-polarized light is reflected by the PBS film. Each port includes a polarizing beamsplitter that spatially separates an input beam into two orthogonally-polarized beams, a reflective prism that reflects one of the beams so that they become parallel, and two electro-optic retarders (or polarization rotators) that change the polarization of the beams so that both have the same polarization exiting the port.
However, the port configuration used by Meadows has several significant disadvantages. This approach is needlessly complex and therefore is more expensive and more difficult to maintain precise optical specifications. In addition, this approach has significant technical limitations in terms of polarization mode dispersion and polarization-dependent losses due to the non-parallel beam paths through each port.
3. Solution to the Problem
The present invention employs a polarization-dependent routing (PDR) element formed by joining a plurality of prisms to create a substantially X-shaped interface having a PBS coating. Each input/output port has a birefringent element and a polarization rotator. This optical routing structure provides polarization-independent and low cross-talk switching over a wide operating range of temperatures and wavelengths. This approach retains the switched signals in optical format and preserves their optical properties.
In addition, the present invention offers a significant advantage over Meadows in that each port maintains a substantially parallel beam path, thereby providing reduced polarization mode dispersion and polarization-dependent losses. The present invention is also simpler, less expensive, and is easier to fabricate than Meadows design.